[Detailed Description of the Invention]
1. Technical Field of the Invention
The present invention relates to a fixing device for use in an image recording apparatus of the electrophotographic type such as an electronic copying machine or a facsimile.
2. Prior Art
Conventionally, in an image recording apparatus of the electrophotographic type such as an electronic copying machine or a facsimile, there is widely used a fixing device in which a sheet having a toner image transferred thereto is passed through a nip portion between a pair of rollers consisting of a fixing roller and a compression roller to thereby fuse the toner image to the sheet by means of heating by the fixing roller and compression by the two rollers.
Generally, in this fixing method, since the toner image fused to the sheet comes into contact with the fixing roller, a roller having a fluorine resin coating layer with high mold release characteristics is used. However, even if such a fixing roller is used, the fused toner easily adheres to the surface of the fixing roller because it is soft and highly viscous, and the sheet may wind around the fixing roller. Thus, there is normally adopted a method to prevent the sheet from winding around the fixing roller with the provision of a forced peeling device using such a peeling claw as described below.
FIG. 10 is a schematic structural view showing a conventional fixing device having a forced peeling device using a peeling claw.
As shown in FIG. 10, this fixing device comprises: a fixing roller 1 incorporating a heater 2 therein, which rotates in a direction indicated by an arrow A; a compression roller 6, which rotates in a direction indicated by an arrow B in contact with the fixing roller 1; and a peeling claw 10, whose end edge comes into contact with the surface of the fixing roller 1 to peel a sheet P, which has passed through a nip portion N, away from the fixing roller 1, provided downstream of the nip portion N where the fixing roller 1 and the compression roller 6 contact one another with respect to the direction of rotation of the fixing roller 1. As the compression roller 6, a rubber roller is normally used, and is arranged so as to press against the fixing roller 1 at predetermined pressure.
As the peeling claw 10, there has conventionally been used a peeling claw obtained by molding heat-resistant resin such as polyimide and polyphenylene sulfite and finishing its tip end into a sharp shape. Such a peeling claw 10 is arranged such that it is pressed against the surface of the fixing roller 1 using a spring. The width of the end edge of the peeling claw 10, which comes into contact with the surface of the fixing roller 1, is nearly 2 mm, and a plurality of the peeling claws 10 with narrow widths are normally arranged in the axial direction of the fixing roller 1. Since the peeling claws 10 only partially come into contact with the surface of the fixing roller 1 in this way, uneven pressure due to the peeling claws 10 is applied onto the surface of the fixing roller 1 in the axial direction thereof, and as a result, the surface may be partially worn or scratched. Also, in a case where the sheet hooks on any one of the plurality of peeling claws to cause winding of the sheet, the adjacent peeling claws may receive an unusual force caused by the sheet to be strongly pressed against the fixing roller 1, or to be deformed, thus seriously scratching the surface of the fixing roller 1 or causing partial wear.
In order to solve such problems, there is disclosed in Japanese Published Unexamined Patent Application No. Sho 59-188681 a fixing device for peeling a sheet by means of the following plastic peeling sheet.
FIG. 11 is a schematic structural view showing a conventional fixing device having a forced peeling device using a plastic peeling sheet.
As shown in FIG. 11, this fixing device comprises: a fixing roller 1 incorporating a heater 2, which rotates in a direction indicated by an arrow A; a compression roller 6, which rotates in a direction indicated by an arrow B in contact with the fixing roller 1; and a peeling sheet 11, whose end edge comes into contact with the surface of the fixing roller 1 to peel a sheet P, which has passed through a nip portion N, away from the fixing roller 1, provided downstream of the nip portion N where the fixing roller 1 and the compression roller 6 contact one another with respect to the direction of rotation of the fixing roller 1. For the peeling sheet 11, there is used a plastic sheet having a thickness of 0.05 mm or more, flexural modulus of elasticity of 10.sup.3 kg/cm.sup.2 or more and a melting point of 150.degree. C. or higher, and it is arranged so that its sharp end edge comes into uniform contact with the entire surface of the fixing roller 1 in the axial direction therefor.
[Problems to be Solved by the Invention]
For example, during formation of a monochrome image, peeling can be performed without any problems in the case where a toner image immediately after fixing is comparatively thin and is highly viscous like. However, in the case where a toner image immediately after fixed is comparatively thick and has high adhesion when heated at high temperatures by the fixing roller like during formation of a color image, there may occur a phenomenon that a large quantity of toner has adhered to a fluorine resin layer on the surface of the fixing roller, and an excessive peeling force acts on the end edge of a thin-film peeling sheet, so that the tip end thereof is plastically deformed and curls up, and as a result, the sheet cannot be peeled. Also, at the same time, the tip end of the sheet may also be damaged seriously to curl up, to become wavy, or to cause paper jamming. Particularly in the case of fixing of a full-color image, since toner of four colors, namely magenta, yellow, cyan and black, is used, a non-fixed toner image formed by superposedly transferring a larger quantity of toner than at the time of fixing a monochrome image must be fixed, and a great peeling force is required on peeling. Further, in the case of fixing a color image, it is necessary to cause the toner to develop colors sufficiently, and for this end, the toner must be sufficiently heated and fused. Therefore, since the toner immediately after it passes through the nip portion has low viscosity, an increasingly greater peeling force will be required.
If the peeling sheet has a thickness of, for example, 200 .mu.m or more in order to cause the peeling sheet not to be deformed even if a great peeling force is applied to the peeling sheet, it will be possible to prevent the peeling sheet from being deformed. However, if the edge portion at the end edge of the peeling sheet has a thickness more than several times larger than the thickness of the sheet to be peeled, it is impossible to stably peel the sheet. Also, if the thickness of the peeling sheet is made excessively thick, the greater flexural rigidity thereof may damage the fixing roller.
In the case where a plurality of peeling claws, which have most frequently been used on fixing a conventional monochrome image, are used in place of the peeling sheet, the above-described problems in the peeling sheet will not occur, but a toner image after fixing is forcibly peeled away by the peeling claws, and therefore, the toner image is damaged by the peeling claws to easily cause image defects. Therefore, the color image is hardly peeled forcibly by the peeling claws.
Under such circumstances, a self-stripping method is often adopted for fixing a color image. The self-stripping method is a peeling method whereby the sheet is arranged to be naturally peeled away from the fixing roller by means of the rigidity of the sheet and the elasticity of the fixing roller instead of using any forced peeling device using peeling claws, a peeling sheet or the like. As a method for implementing this self-stripping method, in the case of fixing a color image, there is widely adopted a method normally in which there is used a fixing roll roller having an elastic layer made of silicone rubber having higher mold release characteristics than fluorine resin, formed on the surface of the roll core, and in which a comparatively large quantity (10 mg or more/A4-size sheet) of oil is always supplied to the surface of the elastic layer.
However, a conventional fixing device which achieves self-stripping has the following various problems:
(1) The reliability of the fixing roller may be reduced by various causes such as wearing of an elastic layer made of silicone rubber on the surface of the fixing roller or deterioration of mold release characteristics or an elastic layer deterioration rated by oil permeated into the fixing roller.
(2) It is inferior in maintainability because oil must be periodically replenished, and is not suitable for a small-sized copying machine or a printer.
(3) Oil easily remains on a copy, which easily deteriorates the touch-up ability with a ball-point pen or in ink.
The present invention has been achieved in the light of the above-described state of affairs, and is aimed to provide a fixing device having a peeling sheet, capable of stably peeling without causing any damage to the image, the sheet and the fixing roller.